Extrusion apparatuses including extrusion heads for producing tread strips for motor vehicle tires are known. One such arrangement is disclosed in German Patent Specification No. 3212157. Such an arrangement comprises an extrusion head having a fixed central portion and upper and lower portions which are mounted so as to be pivotable towards and away from the central head portion. Flow channels for the mixtures being used to form the tread strip are defined between the central head portion and upper and lower head portions respectively. The arrangment further includes locking members which are hydraulically actuated. These locking members project laterally outwardly, at right angles to the direction of flow of the mixture through the head, and engage in lateral portions of the head. A further locking system is also provided to provide high-pressure sealing of the upper and lower head portions respectively against the central portion.
Such an extrusion apparatus is used to produce flat profiles and has from two to four screw presses which are disposed above one another in a vertical plane. The presses jointly convey the various mixtures into the extrusion head. Such an extrusion apparatus is used, for example, in the tires manufacturing industry when unvulcanised tread strips or side walls for vehicle tires are to be produced in a single operation from a plurality of rubber mixtures having different properties of wear resistance, flexibility and/or colouration. In such an extrusion apparatus, the extrusion head comprises a plurality of component parts. Since extremely high pressures occur within the extrusion head, appropriate locking arrangements must be provided.
Such locking is effected by means of trapezoidal wedges which are hydraulically displaceable from each end of the upper and lower head portions. The wedges engage in rectangular apertures formed in lateral portions of the head. This locking arrangement serves to prevent rubber from emerging from the butt joints between the head portions.
The pressure of the material in such extrusion heads may be as high as 300 bars. In other words, pressures of up to 600 t may act on the respective inner surfaces of the upper head portion and fixed central head portion. Because of these high pressures, it is extremely difficult to ensure that the joints of an extrusion head are sealed.
A locking system, of the type described in German Patent Specification No. 3 212 157, is suitable for use in extrusion heads which operate at very low pressures.
When the head is to be closed, that is to say, moved into its inoperative position, the upper head portion and the lower head portion are both displaced towards the fixed central head portion. Subsequently, the wedge-shaped locking members are hydraulically displaced outwardly at each end of the pivotable head portions into rectangular apertures formed in the lateral portions of the head. The locking wedges are forced into the apertures as far as possible, the depth of insertion being dependent upon the frictional forces between the wedges and the apertures. Consequently, due to the wedge effect, a strong closure force is produced.
When the mass pressure in the flow channels exceeds a certain value, approximately 200 bars, however, material emerges from the joints between the head portions, with the result that the operation of the extrusion head is considerably impaired.
An increase in the locking force, or more intensive pressing of the head portions towards one another, during the operation is not possible. This is because, using a wedge-type locking arrangement, the wedges cannot be displaced any further into the rectangular apertures in the lateral head portions due to the fact that the frictional forces increase with increasing mass pressure. In consequence, the position of the wedges in the apertures formed in the lateral head portions cannot be changed in the event of an increase in mass pressure in the flow channels. Hence, if a predetermined mass pressure is exceeded, further sealing of the head portions is not possible.